Immediately upon completion of the spell, and once per round thereafter, you may call down a falling stone that weighs 200lbs and falls from a height of 50 ft (dealing 5d6 damage) The falling stone lands on whatever target point you choose within the spell's range (measured from your position at the time). Any creature in the target square is affected. The Stone falls from greater heights depending on the users caster level. Every additional level beyond fifth, the stone falls an additional 10ft. (no matter how high it falls from, the maximum falling damage of an object is 20d6)

You need not call a falling stone immediately; other actions, even spellcasting, can be performed. However, each round after the first you may use a standard action (concentrating on the spell) to call a stone. You may call a total number of stones equal to your caster level (maximum 15 stones).